GSA Connects 2021 in Portland, Oregon

Paper No. 88-7
Presentation Time: 9:00 AM-1:00 PM


MAHAN, Shannon, U.S. Geological Survey, Box 25046 Federal Center, Denver, CO 80225

About 1.25 million years ago a volcanic eruption in New Mexico’s Jemez Mountains created the Valles Caldera. The Valles Caldera is particularly susceptible to erosion induced by wildfires because of its steep slopes. Gullies that feed the bottom grassland locations are reactivated after a fire season when protective vegetation is burned off and the slopes of the caldera are subjected to increased erosion. In July 2011, the Las Conchas Fire, started by an electrical power line on nearby private land, burned 120 km2 of the Valles Caldera National Preserve causing enormous erosion in a short amount of time.

A 3 m exposure was found in a gully, after the 2011 fire, which revealed alternating light and dark bands of material (15 – 25 cm in width). The abrupt and unusual nature of the contact between different colored material strongly suggests deposition events related to either fire cycles or heavy precipitation events on a fire-ravaged landscape. Two competing theories for the formation of the layers are that they reflect the fire history of the caldera by directly transporting and depositing charcoal or burned sediment or that these layers are part of an organic soil formation in a local wet spot (or marsh) that is periodically buried by alluvium or debris flows after fires. Fifteen OSL samples of the dark bands and three OSL samples of the light bands were measured for quartz OSL. When dated with OSL, material near the bottom is about 7,000 years old and material near the top is about 2,500 years old but there is no apparent pattern to the formation of dark bands.

The quartz was unexpected since the majority of the local sediments are sourced from tuff, although there are a few local exposures of sandstone near the top of the Preserve rim. Luminescence characteristics show that it is blown in and incorporated into the sediment. Particle size analyses reveals that both the light and dark sediment is 80% sand, 5% clay and 15% silt, with some variations, but no large swings, which is puzzling if the darker bands are organic soil, since one would expect a finer grain size. Elemental concentrations reveal no substantial difference in major or minor elements between light or dark layers. A young mollisol was sampled further upstream, along with a modern marsh deposit, and reveals that the darker layers are almost certainly mollisols. This presentation will focus on why these mollisols are unusual and what they indicate about past climatic events in Valles Caldera.